1. Field of the Invention
The present invention relates to an imaging optical system to be mounted on an imaging apparatus using an imaging device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), a digital still camera, a camera-equipped portable telephone, or a personal digital assistance (PDA), and more particularly relates to a bifocal imaging optical system in which the focal length can be switched over to a wide-angle side and a telephoto side.
2. Description of Related Art
In a recent imaging device such as a CCD or a CMOS, miniaturization and increase of pixels are highly advancing. Therefore, also the body of an imaging apparatus, and lenses mounted thereon are requested to be further miniaturized and have a higher performance. As an imaging lens to be mounted on, for example, a camera-equipped portable telephone, a system having a simple lens configuration which is allowed to have only two or three lenses by using many aspherical surfaces is known. When such a simple lens configuration is employed, it is difficult to configure the imaging lens as a zoom lens, and the imaging lens is inevitably configured as a single-focal lens. However, a request for, also in a portable imaging apparatus, performing a telephoto imaging operation in addition to an imaging operation at a normal field angle is increasing. When the number of lenses is increased, it is possible to configure a zoom lens. In this case, however, the cost is increased, and also the size of the whole configuration is increased. Alternatively, a method in which the focal length is changed by externally attaching an attachment lens to the main lens system may be employed. In the method, however, a further optical system must be added to the main lens system. Therefore, the size of the configuration is increased, and hence the method is not suitable for a portable imaging apparatus.
JP-A-2005-265950 discloses an invention of a bifocal lens which does not have a configuration where an attachment lens is added to the main lens system, but has a configuration where a part of the main lens system is selectively interchanged to switch over the focal length between a wide-angle side and a telephoto side. The bifocal lens disclosed in JP-A-2005-265950 includes two lens groups of a first lens group and a second lens group, sequentially from the object side. The second lens group consists of a second group on a wide-angle side and a second group on a telephoto side. In a wide-angle side, a main lens system on a wide-angle side is configured by a combination of the first lens group and the second group on a wide-angle side, and, in a telephoto side, a main lens system on a telephoto side is configured by the first lens group and the second group on a telephoto side. In this way, the focal length is switched between the wide-angle side and the telephoto side by replacing the lens groups on the image side with each other. As a specific example, JP-A-2005-265950 discloses a configuration in which the first lens group consists of two lenses, and the second group consists of two or three lenses.
In the lens disclosed in JP-A-2005-265950, the main lens system is partly interchanged, and therefore it is possible to realize size reduction as compared with the case where an attachment lens is added to the main lens system. However, the bifocal lens disclosed in JP-A-2005-265950 employs the method in which the lens groups on the side adjacent to the imaging surface are interchanged with each other, and hence there is a possibility such as that dust adheres to the imaging surface with the interchange of the lens groups. Usually, a cover glass is placed in front of the imaging surface. In such a case, there is a possibility such as that dust adheres to the cover glass. The example disclosed in JP-A-2005-265950 has a large total number of lenses for an imaging lens to be mounted on a camera-equipped portable telephone or the like, and therefore is insufficient from the viewpoint of miniaturization.